As more than $113 billion worth of home equity has vanished from South Florida’s housing market in the past five years, the number of homeowners with mortgages that are larger than the values of their properties has become enormous. More than 300,000 South Florida mortgages—or 43 percent of them—are currently underwater, the highest level in decades, if not ever. That’s about four times the number of homes in foreclosure.

The underwater problem has been a thorn in the side of a housing market plagued with tight credit, record-high foreclosures and high unemployment. It has contributed to a deluge of loan modification requests, pushed up the foreclosure rate and helped revive a once-taboo exit strategy—the strategic default.